1. Field of the Invention
The present invention generally relates to positioning of a workpiece and, more particularly, to a method and apparatus for repetitively positioning a workpiece with high precision.
2. Description of the Prior Art
Many processes require high accuracy of positioning of a workpiece, particularly in connection with a previously determined pattern which controls the operation carried out on the workpiece, for example, exposure of a pattern onto a radiation sensitive material, directly applying a pattern of material, such as stencilling, or virtually any operation involving automation or robotics. Consequently, many sophisticated techniques have been developed for the purpose of achieving accurate positioning with the degree of precision required.
Generally, such techniques involve some optical process and manual alignment, often with the use of a microscope, if warranted by the required accuracy. It is often possible, after coarse manual positioning, to increase the effective accuracy by sensing the location of registration points and calibrating the operation to the detected position of the workpiece.
However, in some manufacturing operations, it is necessary to perform operations on a workpiece on different machines, requiring the workpiece to be precisely aligned with each machine. This is the case with multi-layer ceramic (MLC) or multi-layer module (MLM) structures where a workpiece or "green-sheet" must have a conductive pattern with high feature density applied to it by screening a paste through a stencil. The stencil and resulting pattern must be precisely aligned with perforations in the green sheet which form "vias" for the purpose of creating connections between or through layers of the finished device. After depositing the pattern, other processes requiring a similar degree of positional precision, such as automated testing may be carried out on the separate layers. Finally, the layers must be precisely positioned in a stack for sintering.
It can readily be appreciated that even this final alignment step, alone, requires a manual alignment procedure for each layer of the device in addition to the other alignment steps which may be required for each layer. Therefore, it can be readily understood that the number of alignment procedures which may be required is a major cost factor in the fabrication of MLM and MLC modules and many other types of devices.
This limitation is by no means unique to these types of electronic circuit modules and arrangements for providing registration mechanisms, typically with mating pins and apertures, to eliminate some of the manual alignment steps are known for manufacturing processes for other types of products. However, MLM and MLC modules are representative of types of structures to which such registration mechanisms are not easily applicable. The principal reasons for this difficulty are that MLM and MLC modules are of a relatively expensive material and of high density. The expense of the "footprint" of any known registration arrangement on a lamina of such material would be prohibitive and would also reduce the effective integration density of the completed modules. The small amount of margin space on the workpiece and the fragility of the workpieces limits the ability to attach ancillary structures thereto in a way which will preserve the alignment accuracy available. Further, the accuracy of alignment available from known registration arrangements is not comparable to or compatible with the dimensional resolution of the screening patterns and present a design trade-off between potential degree on miniaturization and the cost of manufacturing devices where manual alignment procedures limit efficiency and manufacturing throughput. Moreover, known registration mechanisms do not readily provide for accommodation of alignment of a large plurality of layers of a layered structure while maintaining alignment accuracy.